Angular 4 HTTP POST with windows authentication preflight issue

I am using angular 4 with ASP.NET web api 2 on backend with windows authentication enabled. I have one post and one get api. GET works fine. But for POST I am getting this error

Response for preflight has invalid HTTP status code 401

Which means I am getting unauthorised error for OPTIONS method which I know should not be the case because, OPTIONS method is never called with user credentials.

in angular I am calling api as follows

      let requestOptions = new RequestOptions({
      withCredentials: true,
      headers: new Headers({
        'content-type': 'application/json'
      })
    })
    this.http.post("someapiendpoint", JSON.stringify(data), requestOptions).subscribe(data => {
      debugger;
    });

I have set all CORS header in my web config as follows

        <add name="Access-Control-Expose-Headers " value="WWW-Authenticate"/>
        <add name="Access-Control-Allow-Origin" value="http://localhost:4200" />
        <add name="Access-Control-Allow-Headers" value="accept, authorization, Content-Type" />
        <add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE, OPTIONS" />
        <add name="Access-Control-Allow-Credentials" value="true" />

Also I have following in my authentication block of web.config

             <authentication>

                <anonymousAuthentication enabled="false" userName="" />

                <basicAuthentication enabled="false" />

                <clientCertificateMappingAuthentication enabled="false" />

                <digestAuthentication enabled="false" />

                <iisClientCertificateMappingAuthentication enabled="false">
                </iisClientCertificateMappingAuthentication>

                <windowsAuthentication enabled="true">
                    <providers>
                        <add value="Negotiate" />
                        <add value="NTLM" />
                    </providers>
                </windowsAuthentication>

            </authentication>

I tried removing

headers: new Headers({
            'content-type': 'application/json'
          })

Following is my controller code for web api

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using System.DirectoryServices;
using System.DirectoryServices.AccountManagement;

namespace Backend.Controllers
{
    public class DemoController : ApiController
    {
        Entities db = new Entities();
        public DemoController ()
        {
        }

        [HttpPost]
        [ActionName("postevent")]
        public IHttpActionResult PostEvent(Demo data)
        {
            db.Demos.Add(data);
            db.SaveChanges();
            return Ok();
        }

    }
}

from my angular code but it did not help. I also tried to replace it with 'text/html' but that did not work too. Can some one tell me what wrong am I doing. As I assume it should work fine if it is working for GET.